Electromagnetic analysis of concrete tensioning wires

ABSTRACT

Discloses methods to perform magnetic testing of tensioning elements in a pre-stressed concrete cylinder, such as a pipe or water reservoir and testing apparatus. The apparatus includes magnetic flux production means and detector means disposed proximal to a surface of the cylinder in a plane in common with the magnetic flux production means that is orthogonal to an axis of the cylinder. The apparatus operates over a range of low frequency signals, for example, between 20 and 300 hertz or a pulse. Output of the inspection apparatus includes a signal and distance plot showing the results of testing a cylinder at one or more frequencies. In accordance with another method of analysis, a characteristic of the phase of the output over distance is plotted, including the phase or representations of the in-phase or quadrature components of the received signal in relation to the driving signal.

FIELD OF THE INVENTION

[0001] This invention relates to a method of non-destructive inspectionof concrete conduits and cylinders, such as for example water pipes andwater reservoir vessels, which are reinforced with metal wires. Theinvention also relates to apparatus for carrying out such inspections.

BACKGROUND OF THE INVENTION

[0002] There are many wire-reinforced concrete structures in use tocontain or to conduct pressurized fluids, for example forming conduitsin piping systems for water or forming water reservoir vessels. Typicalconcrete conduits are formed of concrete pressure pipe. Concretepressure pipe consists of a thin steel cylinder, over which a layer ofconcrete is cast. Metal reinforcing wires are wound helically, eitherdirectly onto the metal cylinder or onto a layer of concrete cast on thecylinder. Often, a second layer of concrete is cast over the metalreinforcing wires. The exterior of the pipe is then finished with alayer of mortar.

[0003] Concrete vessels used for water storage (as for example tocontain water for distribution) are usually cylindrical incross-section, although they are occasionally oval in cross-section. Thevessel is wrapped around its circumference with wire to providecompressive force to the concrete of the reservoir to support the watercontained within the vessel forming the reservoir. Typically, the wireis of circular cross section, although other cross-sections (e.g.rectangular) are known as well.

[0004] The purpose of the reinforcing wires is to keep the concrete thatthey overlie in compression. Over time, the wires may corrode andeventually break. When this happens, it is possible that a rupture ofthe concrete conduit or reservoir vessel will occur, leading to escapeof the pressurized fluid which it contains.

[0005] It is very expensive to replace an entire conduit or reservoirvessel. Therefore, it is preferred to carry on some sort of inspectionprocedure, to determine where wires have broken. This permits remedialwork to be carried out only in locations that need it.

[0006] Prior techniques of inspection have not been completelysuccessful. Some work has been done with remote eddy field currentdevices, and U.S. Pat. No. 6,127,823 of Atherton has proposedsimultaneously using remote eddy field effects and transformer couplingeffects for inspection. However, as admitted in that patent, theinterpretation of the test results is complicated. Further, because thedevice of the Atherton patent preferably has a spacing of two to threepipe diameters between its exciter coil and its detector coil, it is notsuited to detecting wire breaks near the ends of the pipeline, i.e.within two to three pipeline diameters of the end.

BRIEF DESCRIPTIONS OF THE INVENTION

[0007] According to the invention, an inspection device is provided forconcrete pipes or vessels having a cylindrical wall reinforced withwires wound around the wall, or concrete vessels having an oval wallwith wires wound around the wall. The device has one or more detectorsproximal to the wall to be inspected. The detector can be inside oroutside the wall. When the inspection device is used for inspectingpipelines, the detector is preferably inside the wall, attached to avehicle which can be pulled through the pipeline.

[0008] In one embodiment, the detector is a coil having an axis parallelto the axis of the pipeline, and with an edge proximal to the wall ofthe pipeline. In a preferred embodiment, there are two detectors,axially spaced from each other. Preferably, where the detectors arecoils, the detector coils have a diameter considerably less than thediameter of the pipeline being examined, and more preferably, not morethan one-third of the diameter of the pipe being examined.

[0009] In another embodiment, the detector is a non-coil detector ofelectromagnetic fields, preferably a giant magneto resistive (GMR)sensor. Preferably, the detector comprises three GMR sensors, with theiraxes of sensitivity to magnetic flux orthogonal to one another. Themagnetic flux in the direction desired to be measured (for example,along an axis parallel to the axis of a cylindrical pipeline or vessel)is measured by measuring the flux in the three orthogonal directionsrepresented by the three detectors, and resolving the vectors todetermine the flux in the desired direction.

[0010] In one manner of operation, the invention provides a driver coilto create an electromagnetic field, which creates a current flow throughthe wires forming part of the wire-wound concrete pipe or vessel. Thevoltage and other effects induced by this current in a detector are thenmeasured.

[0011] Preferably, the driver coil has its axis orthogonal to thedetectors, which may be radial to the pipe or vessel in one manner oforientation of the driver coil in relation to a cylindrical pipe orvessel being inspected. The axis of the driver coil will be discussedwith relation to a cylindrical pipe or vessel, which is the normal case.If a pipe or vessel has an oval cross section, the two axes are parallelto one another. In that case, the term “the axis” used herein meanseither of the parallel axes.

[0012] It is preferred that the axis of the driver coil lies in a planeextending across the pipe or vessel, that is transverse to an axis ofthe pipe, and intersecting the detector. Where there are two detectors,the axis of the driver coil is preferably in a plane at right angles toan axis of the pipe and intermediate the two detectors. This has theadvantage that there is no separation along the axis of the pipelinebetween the detector and the driver. This permits measurements to betaken up to only a few centimetres of the end of the pipe, which is notpossible with apparatus where an axial separation must be maintained.Although not preferred, it is possible to use the invention with anaxial separation along the pipe between the detector and the drivercoil. Distances of up to 3.05 m. (10 feet) separation in a 6.1 m. (20foot) diameter pipe have been found to work. However, such axialseparation has no benefit, requires a longer mount for the equipment,and prevents taking readings near the ends of pipes.

[0013] In one embodiment, the detector is offset from the driver coilalong an inner surface circumference of the pipe. The detector may bediametrically opposite the pipe from the driver coil. Where the detectoris a coil, the axis of the detector coil is preferably parallel to theaxis of the pipe. It is possible to have a driver coil that is notcompletely diametrically opposed to the detector, but it is preferredthat the radius along which an axis of driver coil is, should at leastbe on a side of the central axis of the pipe that is remote from thedetector. For large diameter pipes, such as 6.1 m. (20 foot) diameterpipes, it is preferable not to have the driver coil diametricallyopposed from the detector, but circumferentially offset from it, toreduce the length of the equipment mounting boom on which the detectorand driver coil are mounted.

[0014] In one method of operation, the invention provides a driver coilto create an electromagnetic field, which creates a current flow throughthe wires that wrap a concrete cylinder, such as a water reservoir or avery large diameter pipe. The voltage and other effects induced by thiscurrent in a detector located proximal to an exterior surface of theconcrete cylinder remote from the driver coil are then measured.

[0015] The detector is remote from the driver coil along an outersurface circumference of the concrete cylinder. The detector may bediametrically opposite the cylinder under test from the driver coil. Forlarge diameter cylinders, for example 6.1 m. (20 foot) diameter pipes orwater reservoirs of even larger diameter, if the driver coil is notdiametrically opposed from the detector, it is circumferentially offsetfrom it. Where the detector is a coil, the axis of the detector coil isparallel to the axis of the cylinder under test.

[0016] There can be appreciable interference to the signal produced bythe detector through direct magnetic flux coupling between the driverand the detector, for example, the magnetic flux formed within the pipe,between the detector and the driver. To counter this, it is preferableto orient the detector axis to be orthogonal to the driver axis. Furtherreduction of the direct magnetic flux coupling between the driver andthe detector can be obtained by placing a substance of highpermeability, which shields magnetic flux, in a position to block orsubstantially attenuate such magnetic flux. Mu-metal is a metal alloythat is expressly built to prevent passage of magnetic force, so ashield of mu-metal is preferred.

[0017] In a particularly preferred embodiment of the invention, adetector device according to the invention is mounted on a vehiclemovable through the pipe. The vehicle is provided with a means fordetermining its location or distance of travel precisely. The vehicleproceeds down the pipe, while logging information from the magnetic pipeinspection and location or distance information. The locationdetermination means provides a precise location, so that the informationthat is received about the state of the wires can be correlated to aparticular location along the pipe. Optionally, the vehicle is alsofitted with means to propel it through the pipe, and hydrophone means,which can carry out an acoustic examination of the walls as the vehicleis passing it through.

[0018] The vehicle is preferably sized so that, in a large pipeline, itcan be placed in the pipeline through inspection ports, which are spacedalong the pipeline. The vehicle can also be stopped at such inspectionports for the recharging of batteries and removal of recorded data. Ifdesired, a whole or partial analysis can be done at the vehicleprogresses through or traverses the pipe. The results can be displayedgraphically to the operator. Alternatively, the data can be removed andanalyzed at a remote location.

[0019] In one of its aspects, the invention provides an inspectionapparatus for detecting discontinuities in spirally wound metallic wiresembedded the wall of a concrete pipe, comprising a detector forproducing an output responsive to a magnetic flux and a driver means tocreate magnetic flux and located not more than one pipe diameter alongthe axis of the pipe from the detector. Preferably, the detector is acoil oriented with an axis parallel to the concrete pipe, and the drivermeans is a coil with an axis orthogonal to the to the axis of theconcrete pipe for inducing a current in the wires. The detector isoriented proximal to a surface of the pipe, preferably an insidesurface.

[0020] In a preferred embodiment, the inspection apparatus includesdisplacement sensor means to produce an output representative of atleast one distance of the detector a from a known location and means forcausing the detector to move along a wall of such pipe as well as meansfor storing outputs corresponding to the flux detected by the detectorand the displacement of the detector from such known location.

[0021] In another of its aspects, the invention provides a method ofdetecting discontinuities in spirally wound metallic wires reinforcing aconcrete pipe, comprising providing a driving signal to a driver meanshaving an axis oriented orthogonal to the axis of a concrete pipe anddisposed proximal to an inside surface thereof to generate an inducedcurrent in said wires and providing a detector for producing an outputresponsive to a magnetic flux in a direction axial to said pipe, thedetector being located in close proximity to an interior wall of a pipeand within one pipe diameter of the driver along the axis of the pipe.In accordance with the method, the detector is moved along the wall ofthe pipe, and the output and the location of the detector is recorded asit moves.

[0022] In another aspect, the invention provides a method of testing thespirally wound metallic wires reinforcing the wall of a concrete pipe,by generating a driving signal with a signal generator, providing adetector located in a fixed position relative to the signal generatorand not more than one pipe diameter axially along the pipe therefrom,moving the detector and signal generator along the pipe, detecting asignal with said detector in response to said driving signal anddetermining the location along the pipe where the detector is located atthe time each detected signal is detected. Preferably, the periodicdriving signal is generated at more than one frequency, and the detectorsignal is recorded over a range of locations traversed along a length ofpipe.

[0023] In another of its aspects, the invention provides a method fortesting the spirally wound reinforcing wires embedded in the wall of aconcrete pipe along a length thereof using apparatus including magneticflux production means and magnetic flux detector means disposed proximalto a surface of the pipe, the magnetic flux production means and themagnetic flux detector means in a spaced relationship to the other andaxially disposed within one pipe diameter to the other. The magneticflux production means produces a magnetic field in response to a drivingsignal and the magnetic flux detector means produces a detector signalin response to magnetic flux, location indication means and controlmeans operatively connected to said location indication means, to saidmagnetic flux means and to said detector means. The method comprisesproviding a driving signal of at least one frequency; receiving adetector signal; producing an output representative of the detectorsignal corresponding to the in-phase and quadrature components of thedetector signal in relation to the driving signal; and recording theoutput representative of the detector signal and the location; wherebyat least one output is recorded over a range of locations traversedalong a length of pipe.

[0024] The invention will be further described with respect to thedrawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is s cross-section through a pre-stressed concrete pipe,showing schematically a first form of detector according to theinvention within such pipe.

[0026]FIG. 2 is a cross-section of a similar pipe, showing a secondembodiment of the inventive detector system.

[0027]FIG. 3 is a cross-section through a similar pipe, showing a thirdembodiment of the inventive detector system.

[0028]FIG. 4 is a cross-section through a similar pipe, showing a fourthembodiment of the detector system according to the invention.

[0029]FIG. 5a is a schematic cross-section through a length of pipeline.In order to demonstrate a wire breakage, wires (which would in realitybe concealed behind the metal lining in the view shown) are shown.Further, the drawing is not to scale, and dimensions have been distortedso that detail of wire placement and wire breaks can be shown.

[0030]FIG. 5b is a plot of voltage against distance using the detectorof the invention, on the pipe of FIG. 5a.

[0031]FIG. 5c is a schematic cross-section through a length of pipeline,which is not to scale and has distorted dimensions similar to FIG. 5a,with two wire breakages shown.

[0032]FIG. 5d is a plot representative of an output representative ofthe detector signal phase against distance produced in accordance with amethod of the invention, from an inspection of the pipe of FIG. 5c.

[0033]FIG. 6 is a vehicle designed to pass through a pipe according tothe invention, and having a detector system according to the inventionplaced on it.

[0034]FIG. 7 is functional schematic diagram of the electronic signalelements of an embodiment of the invention.

[0035]FIG. 7a is a functional schematic diagram of two sensors connectedin a common polarity configuration.

[0036]FIG. 7b is a functional schematic diagram of two sensors connectedin a reverse polarity configuration.

[0037]FIG. 8 is functional schematic diagram of the electronic signalelements of a preferred embodiment of the invention.

[0038]FIG. 8a is a graph of a vector output representing the in-phaseand quadrature components of a received signal output from a lock-inamplifier.

[0039]FIG. 9 is a graph showing detector trace plots produced inaccordance with the invention for two exemplary driving frequencies.

[0040]FIG. 10 is a graph showing detector trace plots produced inaccordance with the invention for a plurality of driving frequencies.

[0041]FIG. 11 is a graph showing a detector trace plot of a component ofa detector output produced in accordance with the invention for a singledriving frequency.

[0042]FIG. 12 is a graph showing a plot of a component of a detectoroutput and a corresponding plot of a phase shifted component of adetector output produced in accordance with the invention for a singledriving frequency.

[0043]FIG. 13 is a graph of a vector output representing the in-phaseand quadrature components of a received signal output from a lock-inamplifier transposed by an angle alpha.

[0044]FIG. 14a is a cross section through a pre-stressed concrete pipe,showing schematically a preferred arrangement of the driver and detectoraround the pipe under test.

[0045]FIG. 14b is an elevation view of the pipe and arrangement of FIG.14a, with the protective mortar of the pipe is not shown so that theunderlying structure can be viewed.

[0046]FIG. 15a is a cross section through a pre-stressed concrete pipe,showing schematically an alternate arrangement of the driver anddetector around the pipe under test from the arrangement of FIG. 14a.

[0047]FIG. 15b is an elevation view of the pipe and arrangement of FIG.15a, where the protective mortar of the pipe is not shown so that theunderlying structure can be viewed.

[0048]FIG. 16 is a top view of a pre-stressed water reservoir vessel,showing schematically an arrangement of the driver and detector aroundthe vessel under test.

[0049]FIG. 17 is an elevation of the vessel and arrangement of FIG. 16.

[0050]FIGS. 18a, 18 b, 18 c and 18 d are cross sections through apre-stressed concrete pipe, showing schematically alternate preferredarrangements of the driver and detector disposed about a pipe undertest.

DETAILED DESCRIPTION OF THE INVENTION

[0051] The preferred embodiments of the invention will now be describedwith reference to the Figures. FIG. 1 shows a cross-section through apre-stressed concrete pipe generally indicated as 10. A pre-stressedconcrete pipe of this sort has an inner metal cylinder 11. Dependingupon the type and grade of pipe, either pre-stressing wires are wounddirectly onto the cylinder, or a layer of concrete is cast onto thecylinder, and the pre-stressing wires are wound on the layer ofconcrete. Some pipes also have a layer of concrete cast inside the pipe,separating the metal cylinder from the interior. Other pipes have twolayers of pre-stressing wires, with layers of concrete between them,outside the metal cylinder. Another layer of concrete or protectivemortar is cast around the wires to complete the pipe. Pipes are soldunder the designation ECP, LCP, SP5 and SP12, and are usually designedto meet AWWA standards C301 and C304. All of these types of concretepressure pipe can be examined using the detector of the presentinvention.

[0052] In FIG. 1, pipe is shown as having a metal cylinder 11, wrappedwith wires 12 embedded in concrete 13.

[0053] The pipe inspection apparatus is shown schematically at 15. Theapparatus comprises a detector 16. This detector is preferably a coildetector capable of detecting magnetically induced fields or currents inthe pipe being examined.

[0054] In the embodiment shown, the detector 16 is a coil, which isadapted to receive magnetic flux and convert it into a measurableelectrical current and voltage. However, instead of a coil, any otherdetector of magnetic flux could be used. Another particularly preferredsensor is a giant magneto resistive sensor, or GMR sensor. Such sensorswill be described henceforth in this application as “GMR sensors”.

[0055] When detector 16 is a coil, it is located so that the axis of thecoil is parallel to the centre axis 14 of the pipe. The detector 16 isplaced so that it almost touches the wall of the pipe. It is preferredthat the detector does not touch the wall, as this would impede movementof the detector along the interior surface of the wall. However, the gapbetween the detector 16 and wall of the pipe 10 should be kept as smallas is conveniently possible, having regard for the fact that thedetector is to be moved along the length of the pipe.

[0056] Reference numeral 19 represents a diameter of the pipe, whichpasses through detector 16. At the opposite end of the diameter 19 fromdetector 16, there is a driver coil 17. Preferably the driver coil isdriven with low frequency alternating current, for example from 20 hertzto 300 hertz but may be driven with a pulse. The coil is located so thatits axis is orthogonal to the axis of the pipe being inspected. Thedriver is placed by a wall of the pipe, and, in the preferredarrangement, it is preferable that the driver be disposed as close aspossible to a wall of the pipe. Having regard to the fact that theapparatus will be moved along the pipe, it is not desirable to have thedriver 17 drag against the wall of the pipe in operation of theapparatus.

[0057] Optionally, a shield 18 of a high permeability material, that ismaterial that impedes the passage of magnetic flux therethrough, isplaced between the detector 16 and the coil 17. A suitable material ismu-metal. The purpose of the mu-metal shield is to prevent magnetic fluxpassing through the contents of the pipeline directly to the detector 16from the driver coil 17. It is intended that the primary signal pickedup by the detector 16 should be the signal that is made by an inducedcurrent in the wires 12, because of the driver coil 17. A signal causedby magnetic flux in the contents of the pipeline would add noise to thissignal. In a very large pipeline, particularly when the pipeline hasbeen drained for inspection, the signal passing directly from the driver17 to the detector 16 is often insignificant (large pipelines, for watertransmission, are often several meters in diameter). However, where thepipeline is smaller, and particularly when the pipeline is filled withwater, direct signals through the contents of the pipeline may be aproblem, so the shield 18 is desirable in such circumstances. Thedesirability of the shield 18 can be determined by doing samplemeasurements with and without the shield, to see whether the shieldmakes an appreciable difference in the measurements.

[0058]FIG. 2 is a view similar to that of FIG. 1, showing a secondembodiment of the inspection device. Reference numerals, where they arethe same as used in FIG. 1, designate subject matter the same as in FIG.1 in this and all subsequent figures.

[0059] In the embodiment of FIG. 2, the inspection device is generallyshown as 20. It has detector means provided by two detectors 21 and 22,for example coils, which are spaced from one another along a common axis23 by a distance less than the diameter 19 of the pipeline. Preferably,the spacing of the detectors is small, such as from about 7.5centimetres to preferably not more than half the diameter of thepipeline.

[0060] Alternately, the detector means is a giant magnetoresistive (GMR)sensor which has an axis responsive to magnetic flux analogous to anaxis of a coil detector. A GMR sensor provides an output which is achange in resistance that is induced in the GMR sensor by magnetic flux.To provide an analogous structure to the two detectors 21 and 21 of FIG.3 or 3, the detector means is a pair of GMR and the output is a changein resistance induced in the GMR detects by magnetic flux.

[0061] Where it is desired to resolve a magnetic field in threedimensional space, three detectors oriented along each orthogonal axiscan be used. For example, where the detector means has three detectors,each oriented to be responsive to a magnetic flux along a correspondingorthogonal axis.

[0062] In FIG. 3, the inspection device 30 has again two detectors 21and 22 on a common axis 23. In this case however, there is a driver coil27, which is not diametrically offset from the two detectors. The drivercoil 27 can be located anywhere on the internal circumference of thepipeline, so long as it is far enough away so that the magnetic fluxproduced by current in the wires of the pipe are distinguishable fromany stray magnetic fields from the driver coil. In this case, the drivercoil is located approximately 45 degrees offset from the verticaldiameter 19 across the pipe as shown by the angle x, and the coil isdisposed radially of the pipe. Particularly in large pipes (for examplethe 20 foot (6.1 m.) diameter pipe mentioned above, it is preferablethat the driver coil be offset from the vertical diameter, even if thedriver coil is not axially offset along the pipe from the detector. Thedriver coil is offset circumferentially from the detector means by anangle x which preferably is at least 10 degrees. Where the driver coilis offset circumferentially from the detector means by an angle x, thecircumferential offset is preferably a distance of at least one meter.This permits having a smaller set of booms on which to mount thedetector and drive coil, and sometimes in large pipes results in acleaner signal.

[0063]FIG. 4 shows a detector with the same arrangement as FIG. 3.However, in FIG. 4 the detector has a driver coil, which is wrappedaround an axis radial to the pipeline and spaced at a distance “I” fromthe diameter 19 between the detectors 21 and 22. The distance “I” isless than one pipeline diameter. Although not shown, magneticallyimpermeable barriers, such as barriers 18 in FIGS. 1 and 2, can beplaced in the line of sight between driver coils 27 or 28 and detectors21 and 22.

[0064] Generally, it is preferred not to offset, along the axis of thepipe, the driver coil from a line 19 extending orthogonal to the axis ofthe pipe to the detector coil or (in the case of two detector coils, aline located midway between the two detectors). If there is no offset ofthe driver relative to the detector, measurements can be made right tothe end of a pipe section. Further, the signal detected is oftenclearer, with less “noise” as induced currents pass through fewer wiresin the pipe before generating the major part of the magnetic fluxproximal to the detector apparatus.

[0065]FIG. 5a shows, in schematic form, a pipeline 60, having a seriesof pipes 61, 70 and 80. These are laid end to end to form the pipeline,and are connected by the well-known bell and spigot system. The pipelineis not shown to scale. Typically, the pipe sections would be of theorder of three meters in length, and pipe diameters would be of theorder of one and one half-two meters.

[0066] Pipe 61 is shown as joined to pipe 70 by a bell 63 which is partof pipe 61. A spigot 71 from pipe 70 is inserted into the bell, andsuitably sealed. Similarly, pipe 70 has a bell 73, into which spigot 81of pipe 80 is inserted and sealed. Each pipe is a concrete pressurepipe, having an internal metallic cylinder (numbered as 64 for pipe 61,74 for pipe 70 and 84 for pipe 80). This is wrapped (either with orwithout an intervening layer of concrete as described) with helicalreinforcing wire. For pipe 61, this wire is 62. For pipe 71, the wire isshown as 72 and for pipe 80, the wire is shown as 82. Only a very fewwires schematically are shown for each pipe. In actual practice, thepipe would be closely wound with such wires, and there could be severallayers of wire, separated by layers of concrete. The wires are overlaidwith concrete or protective mortar to make the pipe.

[0067] In the drawing, a few selected wires are visible. In actual fact,these would not be seen in a cross-sectional view of the pipe, as themetal cylinders 64, 74 and 84 would hide them. However, they are shownfor the purpose of illustrating what happens when there is a break. Abreak is shown in one wire at 85.

[0068]FIG. 5b is a plot of voltage against distance along the pipe, forone detector such as shown at 16 in FIG. 1. Driver coil 17 in FIG. 1 isgenerating a periodic signal at a selected frequency in the rangefrequency of 20-300 hertz, and detector 16 is receiving a voltage.

[0069]FIG. 5b shows a plot of this voltage against the distancetravelled by detector 16 along the pipe. Detector 16 and driver coil 17are rigidly linked, so that they each travel at the same speed.

[0070]FIG. 5b is a plot of voltage (on axis 90 of the plot) againstdistance travelled (on axis 91 of the plot). Only positive voltages areplotted on this plot. As will be seen, the pattern of the plot ofvoltage against distance is that there is a peak, as shown 93, as thedetector traverses each of the bell and spigot connections. In between,there is a relatively flat portion. Thus, while the detector istraversing pipe 61, there is initially a peak 98 as the detector passesover the bell and spigot connection just before pipe 61, then a flatportion 92 as it traverses that pipe length. When it approaches bell andspigot connection 63, 71, the voltage rises again to a peak 93. After ithas reversed that connection and is traversing pipe 70, the voltageagain drops to a relatively flat portion 94.

[0071] After a series of pipes have been traversed, it becomes possibleto determine an average voltage for the peak when a bell and spigotconnection is passed. This average voltage is shown as 95. The voltage95 is the average of peaks 98 and 93. Similarly, it is possible topredict an average voltage when the detector is passing over a sectionof the pipe that does not have a bell and spigot connection. Thisaverage voltage is shown as 96. In the example given, it is the averageof the flat portions 92 and 94.

[0072] In the example given, as the detector 16 traverses the bell andspigot connection 73, 81, a further peak 99 is obtained in the voltage.This peak is approximately the same as peaks 98 and 93, as is expectedfrom the previous peaks for bell and spigot connections. However, afterdropping from peak 99, the voltage first drops approximately to average96 as shown at 97, then rises again as shown at 200, then drops again asshown at 201 to approximately average 96, before rising again to anotherpeak for a bell and spigot connection, as shown 203. The result is a“bulge” 200, which indicates that there is an anomaly in the pipesection being examined. This anomaly is indicative of a broken wire inpipe 80, as is shown in FIG. 5a at 85. (The drawings are not to scale).

[0073] When there is a single detector, it is possible to find thebroken wire with a fair degree of accuracy, as being approximately atthe midpoint of peak 200 in the curve of FIG. 5b. However, the accuracycan be greatly increased by using two sensors, as shown at 21 and 22 inFIG. 2. The two sensors (for example receiver coils) can be very closetogether (for example, about 0.6 cm apart) or can be spaced from eachother by a longer distance, such as for example 60 cm. Generally, thetotal effect of the wire windings is symmetrical upstream and downstreamof the detector coils. However, when there is a discontinuity in thehelically wrapped wires, such as a wire break, this unbalances theeffect, and a very large difference in the signal received at the tworeceiver coils is found. By analyzing the plots of the voltage againstdistance of the two coils, a very precise position can be given for thebreak in the wire.

[0074] In FIG. 5c, a pipe section similar to that of FIG. 5a is shown.The same reference numerals are used to identify the same things. Inthis pipe, there are two broken wires at 75 and 85.

[0075]FIG. 5d is a plot similar to that of FIG. 5c. However, the plot isproduced from the phase relationship of the received signal to thedriver signal which is represented as a phase angle, a voltagerepresentative of the in-phase component of the received signal, avoltage, representative of the quadrature component of the receivedsignal, or a voltage representative of either the in-phase or quadraturecomponent of the received signal translated by a selected angle alpha,all of which are referred to herein as “the detector phase.” A plot ofthe detector phase gives rise to peak and trough patterns, includingpositive and negative values, instead of the peaks shown in FIG. 5b.

[0076] As is shown in the plot, the pattern of the detector phaseagainst distance is that there is an excursion resulting in peak-troughcombinations at each of 98, 93 and 99 as the detector traverses each ofthe bell and spigot joint connections. In between, there is a relativelyflat portion, which approximates the average 96 for the pipe portionsbetween bell and spigot. Thus, while the detector completes traversingpipe 61, there is initially a diverging excursion resulting in peak andtrough pair 98 as the detector passes over the bell and spigotconnection 58, 59 on entering pipe 61, then a substantially flat portion92 (approximating average 96) as it traverses the pipe length. When thedetector approaches bell and spigot connection 63, 71, the plot makes adiverging excursion again to form peak and trough pair 93. After it hastraversed that connection and is traversing pipe 70, the plot againdrops to a flat region. However, in this case, the substantially flatregion is interrupted by a small peak 200 and a small trough 201, whichis a diverging excursion corresponding to wire break 75. When thedetector approaches bell and spigot connection 73, 81, the plot makes adiverging excursion again to form peak and trough pair 99. After thedetector has traversed that connection and is traversing pipe 80, theplot again drops. However, instead of reaching a substantially flatportion corresponding to line 96, it instead ramps down to a trough 97,and then rises to a small peak 203. The ramp and peak excursioncorresponds to wire break 85.

[0077] After a series of pipes has been traversed, it becomes possibleto determine an average of the upper and lower excursion of the peakswhen a bell and spigot connection is passed. These averages are shown asupper and lower lines 95. Each of the upper and lower lines 95 is theaverage of the respective upper or lower peak excursions of the peakpairs, for example, 98, 93, and 99. Similarly, it is possible to predictan average signal when the detector is passing over a section of thepipe, which does not have a bell and spigot connection. This average isshown as 96.

[0078]FIG. 6 shows a vehicle equipped with one embodiment of theapparatus of the invention (in this case the embodiment of FIG. 2). Thevehicle is designed to pass through the pipelines to detect broken wiresand other discontinuities. In FIG. 6, a concrete pipe is generallyindicated as 100. The pipe 100 is formed of concrete 103 having helicalwires 102 as reinforcement. Frequently, the pipe 100 has a steelcylinder 101 disposed interior to concrete 103 providing a steel-linedconcrete pipe. Additional concrete lining may be provided interior tothe steel cylinder 101. Although the pipe as shown in the figure doesnot have a concrete lining within the steel cylinder, the operation isthe same when the pipe does have a concrete lining.

[0079] The pipe is provided at intervals with inspection hatches. Hatch105 is one of such hatches. It has a flange 107, on which a hatch cover106 rests removably. The hatch has an opening of a width shown by thearrow “a”

[0080] The inspection vehicle 150 has a body 170, with wheels 171. Inthe present embodiment, there are eight wheels extending outwardly alongradii of the pipe spaced 90 degrees from each other, with two wheels 171on each radius. Six of these wheels are shown in the drawing. Two othersare behind the body 170. Each wheel 171 is on an axle 172, which ismounted to body 170 by means of a suitable axle support 173. Preferablythe axle support 173 includes springs 174, so that the wheels willdeflect from any discontinuity on the surface of the pipeline.

[0081] Extending from the front of the body 170 of the vehicle, in thedirection that the vehicle will travel within a pipeline, is a supportbar 165, which, as shown, is disposed axially of the pipeline. Connectedto this at right angles is a frame 164, constructed from anon-ferromagnetic material such as aluminium or fibreglass to avoid adriver influence on the detector apparatus via the frame. Frame 164 hasthe detector apparatus mounted on an end remote from the driver 163. Inthe embodiment depicted, the detector apparatus is a pair of detectors161 and 162 mounted on opposed ends of a crossbar 166. Preferably, thedetectors are co-axial detector coils with their axis parallel to thecentral axis of the pipe. Alternately, the detectors are GMR sensorswhen GMR sensors are used, it is particularly preferred to have eachdetector made up of three GMR sensors with their axis of sensitivityorthogonal to one another, so as to detect the magnetic field from alldirections. The two detectors 161 and 162 are spacedly disposed fromeach other approximately 10 cm apart. As discussed, the preferredspacing of the detectors can be from approximately 0.6 cm to 60 cm,depending on the size of the pipeline and precision desired. Each of thedetectors 161 and 162 is responsive to a magnetic flux in its vicinity.For example, each detector is a coil which has a current induced in itby magnetic flux in its vicinity. Control means, generally referenced byblock 169, are provided in the body of the vehicle to measure thecurrent in each coil, and to record the current measured.

[0082] At the other end of frame 164 is a driver coil 163. This drivercoil is driven by an alternating current in the 1-300 hertz range, or apulse, of sufficient magnitude to induce a current flow in the wires ofthe pipe. A current source 191 is connected to driver coil 163 toprovide the driving current. Magnetic barrier 168, preferably composedof mu-metal, is provided to block stray magnetic fields from extendingthrough the volume enclosed by the pipeline between driver coil 163 andthe detector apparatus, namely detectors 161 and 162.

[0083] A displacement sensor 176 to provide location information of theapparatus as it extends along the length of the pipe is provided, forexample, an odometer. The displacement sensor 176 is connected to one ofthe wheels 171, to provide a displacement measurement based on thedistance travelled by that wheel. Displacement sensor 176 produces asignal indicative of the distance travelled along the pipe, which isrecorded with the recording of the outputs of detectors 161 and 162, toprovide a record of distance travelled. Alternately, instead of being adisplacement sensor, 176 can be a GPS locator or similar device that candetermine and record its location.

[0084] The vehicle can be manually moved through the pipeline by being“walked” by an attendant, or it can be pulled through the pipeline by awire line. However, it is especially preferred to have the vehicleautonomous. In this case, the vehicle can be placed in the pipeline at apoint where there is a suitable opening. The vehicle is made so that itcan be disassembled into parts, which can be handed into the pipelinethrough the inspection port 105 or similar ports. Thus, no singlecomponent of the vehicle has all of its dimensions greater than thedistance represented by arrow “a”. The result is that the vehicle can bepassed into the pipeline in sections when the pipeline is depressurized,and can be assembled. Then, the operators can leave the pipeline andclose off the inspection port, letting the vehicle remain in thepipeline.

[0085] If desired, the autonomous vehicle in the pipeline could have amotor means 195 sufficiently powerful to power it, either against orwith the current of the pipeline (obviously, powering it with thecurrent of the pipeline is preferable, as less power is required). Ifcould also have battery means 194 to power this motor. However, it ispreferred that the vehicle be carried along by the flow of the pipeline.In the present embodiment, there is a deflector 190 mounted on the back(i.e. upstream) end of the vehicle. When the flow of the fluid in thepipeline (usually water) hits the deflector, it pushes the device in adownstream direction (to the right in FIG. 6). An alternate way ofpropelling the device would be by deploying a parachute downstream (i.e.to the right of the device in FIG. 6) to pull it through the pipeline.

[0086] Suitably, the vehicle according to the invention can also be usedto do other types of examination of the pipe as it passes through. Forexample, FIG. 6 shows hydrophone 180 mounted on body 170 of the vehicle.The hydrophone 180 senses sounds in the vicinity of the vehicle as itpasses through the pipe. The data produced by hydrophone may be storedin a data storage device 181 along with location data produced by thedisplacement sensor 176. The hydrophone data can be used to indicatesites of possible leakage and other information as known in thehydrophone art. Other types of sensors can also be mounted on themachine.

[0087] The vehicle can also be equipped with automatic data transfercapabilities. Thus, as the vehicle approaches an inspection port 105, anoperator can trigger it to transmit the data that it has received and anoperator on the other side of inspection port 105 can operate a probe toreceive this data by wireless modem, acoustic modem, or inductivecoupling and/or recharge the batteries as needed. Alternately, as thevehicle reaches an inspection port, a barrier can be placed in thepipeline at the inspection port to stop the vehicle. The line can thenbe depressurized and the inspection port can be opened. The vehicle canthen be examined for its physical condition, the data that it hascollected can be downloaded, and the batteries, which provide the outputgenerated by the AC current of the driver coil can be recharged.

[0088] It is also within the scope of the invention to provide motormeans 195 and battery 194, for use if the vehicle becomes stuck betweeninspection stations, or if the current in the pipeline becomesinsufficient to move it. If desired, this can be triggered by a soundsignal of a predetermined sort, which can be sent from an inspectionport 105 and received by hydrophone 180. There are of course other knownmeans to control movement of the vehicle, as by having it drag a wireline.

[0089] The output signal from such a vehicle can be presented as a graphsuch as that shown in FIG. 5b. It is extremely easy to notice from sucha graph where a wire break has occurred. It is also possible, however,to have initial processing done on the vehicle, so that the vehicle willprepare a supplementary data stream, which generates an exception whenthere is a voltage which is not at the voltage indicated as 95 or 96 inFIG. 5b, and which is not part of the smooth transition between them.Thus, the voltage registered at 200 would be noted as an exception.Thus, a signal would be generated showing each exception, together withthe distance (according to the displacement sensor) at which theexception occurred.

[0090] It is within the invention to use other distance measuring meansother than a displacement sensor 176 mounted on the vehicle. For examplemeans for measuring the velocity of the vehicle and the time that it hasmoved at that velocity are suitable, particularly if there arecalibrating means at inspection ports to provide a calibration as thevehicle passes. A location sensor such as a GPS sensor can also be used.

[0091] Suitably, the vehicle can also have a means to control itsvelocity when it is passing through the pipe. For example, the systemcan, when it does on site processing and indicates that there is ananomalous signal which could represent a wire break, have means tochange the angle of deflector 190, so that it will move more slowlythrough the pipe for a predetermined distance thereafter. Alternately,the deflector can be adjusted so that the device will stop completelybefore a predetermined time, after which the deflector then is moved sothat the vehicle will continue moving through the pipe. Other inspectionmeans, for example hydrophone 180, can be used to provide acousticalsignalling which can be utilized to determine if there is continued wirebreakage occurring at or around a location where there is an anomaloussignal.

[0092] In most pre-stressed water or sewage pipe, fluid flow within thepipe is one to ten feet per second. This is a very convenient speed atwhich to carry out inspection. Inspection with the autonomous vehicle ofthe invention permits the inspection to be done without interruptingflow or emptying the pipe.

[0093] The invention also comprises instrumentation exterior to the pipe(for example at access ports 105, which recognize signals emitted by thevehicle). For example, the vehicle can have a transponder 211, whichresponds to a sound emitted by a fixed location transponder 210, forexample on access port 105, and responds with a sound of its own,thereby giving location information as it passes by inspection ports soequipped. This equipment can also be used for calibration of thedistance measurement as discussed above.

[0094] It is preferred that attachments 164, 165 and 166 are providedwith mechanical damping means, so that mechanical vibration is kept to aminimum, as such vibration can lead to electrical noise which couldeffect the quality of the signal being received.

[0095]FIG. 7 shows a functional schematic diagram of a circuit that canbe used to implement the electronic components of the present invention.An output signal to drive the driver coil 163 of the invention isobtained from output line 200. An amplifier 202 provides sufficientsignal power on output line 200 to produce a magnetic field from drivercoil '163 that is efficacious for the purposes of the electromagneticinspection of the present invention. An opamp is shown as the amplifier202 in the functional schematic diagram. Power transistors or a moreelaborate amplifier circuitry can be used to provide the amplifier 202that will function to produce the signal power for output on output line200. A variable frequency signal generator 204 provides a periodicsignal for input to amplifier 202. Microprocessor 206 provides theparameters of the periodic signal. The parameters define the periodicsignal produced, which is preferably, a sine wave, but can include, forexample, a square wave or a sawtooth wave periodic signal. A sine waveis preferable as it provides a single frequency output signal. A pulsesignal can also be used to obtain a transient response signal at thedetectors. The type of the periodic signal and its repetition rate orfrequency is set by microprocessor 206 over data communications line208. In this arrangement, microprocessor 206 controls the parameters ofthe frequency and type of wave of the periodic signal that is to beproduced by the variable frequency signal generator 204. The parametersmay define a periodic signal that is a range of frequencies, forexample, 20-300 Hz, which are to be continuously produced by the signaltraversed over a given time period, such as a few seconds ormilliseconds. Parameters defining this type of driving signal produce acontinuous frequency sweep over the range of interest repeated for eachsuccessive time period.

[0096] Output from the detector apparatus 16 is received on input line213 where it is supplied to an input of a multiplier 216. A referencesignal derived from the output signal appearing on output line 200 isalso supplied to an input of multiplier 216 via an attenuator 218. Theoutput signal of the multiplier is low pass filtered at 217 and suppliedto input amplifier 212. In this manner, the signal arriving at the inputamplifier 212 is a signal that represents differences between the drivercoil signal and the detector signal. The filtered signal output frommultiplier 216 is amplified by input amplifier 212 and converted to adigital signal by a digital signal processor (DSP) 214. Thus the digitalsignal of DSP 214 is derived from the output of the sensor 16. Whileonly one input line 213 and corresponding amplifier 212 and DSP 214 isshown in the diagram, an additional input line can be provided ifdesired. If there are two detectors, for example 161 and 162 as depictedin the other figures, and each is to provide an independent input to themicroprocessor 206 for processing, then a second path, duplicating inputline 213, multiplier 216, amplifier 212 and DSP 214 is provided. To usetwo detectors to provide a single input signal on input line 213, theoutput from detectors 161 and 162 is coupled together. The detectors canbe coupled in a common polarity or a reverse polarity configuration.FIG. 7a shows the coupling of detectors 161 and 162 together in a commonpolarity configuration. In this configuration, the signal output of thedetector apparatus is coupled in an additive fashion such that the sumof the outputs of each of the detectors 161 and 162 adds to the signalthat is provided on input line 213. FIG. 7b shown the coupling ofdetectors 161 and 162 together in a reverse polarity configuration. Inthis configuration, the signal output of the detector apparatus iscoupled in an subtractive fashion such that the difference of theoutputs of each of the detectors 161 and 162 is the signal that isprovided on input line 213. By using more than one detector, differentlocations and orientations of detectors can be achieved.

[0097] A displacement sensor 176 provides input to microprocessor 206representative of the location of the vehicle in the pipe. When theapparatus of the present invention is placed in a pipe as depicted inFIG. 6, the magnetic coupling between driver coil 162 and the detectorapparatus is manifested by variations in the input signal provided toamplifier 212. Microprocessor 206 receives digital signalsrepresentative of the input signal over data line 220 and performscomputations based on the received digital signals to produce a visuallyperceptible output on display device 220. Preferably, the visuallyperceptible output is graph on a display device 220. Display device 220can be a computer monitor such as a CRT or LCD, or, display 220 can be aprinter that produces a printed output of the signal. The outputproduced may include processing performed on the signal, for example toprovide numeric outputs as graph axis, produce averages traces or changetrace colours. The signal received by the microprocessor 206 can bestored in a data storage device 181, which can be a magnetic disk orother suitable form of storage device such as, for example, a floppydisk or CD.

[0098] Because the output of detector 16 is subject to noise, it ispreferable to use a phase sensitive detector, or a lock-in amplifier, onthe detector output in place of multiplier 216 and amplifier 212. Aphase sensitive detector multiplies the signal received from thedetector with the reference, or transmitter, signal and integrates theresulting product signal to produce a DC signal representative of theamplitude of the received signal. A description of manner of operationand use of a phase sensitive detector and lock-in amplifier may befound, for example, in the publication DSP Lock-In Amplifier modelSR830, Revision 1.5, November 1999 published by Stanford ResearchSystems of Sunnyvale Calif. FIG. 8 shows a functional schematic diagramof a preferred embodiment of a circuit to implement the electroniccomponents of the present invention. The driver configuration is a shownin FIG. 7, however, the input received from the detector apparatus 16 online 213 is supplied to a lock-in amplifier, for example, a model SR830manufactured by Stanford Research Systems of Sunnyvale, Calif. The typeof periodic signal is preferably a sine wave, but can include, forexample, a square wave or a sawtooth wave periodic signal. If othersignals than a sine wave are used, the lock-in amplifier will provide anoutput reference only to the fundamental frequency of the periodicsignal, the higher harmonic components will be discarded. Microprocessor206 controls the parameters of the driving signal. For example, thesweep frequency range and time frame, or a frequency, the type of waveas parameters of the periodic signal that is to be produced by thevariable frequency signal generator 204.

[0099]FIG. 8a shows an output from the detector apparatus 16 is receivedon input line 213 where it is supplied to an input of lock-in amplifier(LIA) 219. A reference signal input to LIA 219 is derived from theoutput signal appearing on output line 200, which may be reduced inmagnitude if needed by an attenuator 218. The LIA 219 provides twooutput signals on 221 and 223, which represent the magnitude and phaseof the AC signal produced by detector 16 at the frequency correspondingto the reference frequency. The output on 221 and 223 can take eitherthe form of an (X, Y) value pair as Cartesian co-ordinates, which definethe in-phase (shown as X on the I axis) and quadrature (shown as Y onthe Q axis) components of the received signal. The received signal mayalso be represented in polar co-ordinates as an (R, Theta) value pair.Using either co-ordinate method, the value pair defines a vectorrepresentative of the received signal in relation to the fundamentalfrequency of the driving signal.

[0100] The value pair produced by LIA 219 is converted to a digital formby DSP's 214 and supplied to microprocessor 206 where it is stored instorage 181. While only one input signal path comprising detector 16 andcorresponding input line 213, LIA 219 and DSP's 214 are shown in thediagram, an additional input path can be provided for each additionaldetector. For example, if there are two detectors 161 and 162 asdepicted in the other figures, and each is to provide an independentinput signal to the microprocessor 206 for processing, then eachdetector would have a signal path comprising input line 213, LIA 219 andDSP's 214.

[0101]FIG. 9 is a graph of an output provided on display 220. Thedetector apparatus produces an output signal while traversing a lengthof pipe, which is stored in storage 181. This data is used to producethe graph of FIG. 9. The graph provides the distance of travel of thedetector apparatus along the length of the pipe is shown as thehorizontal axis of the graph. The vertical axis of the graph representsa voltage level output of the LIA 219 output as received signal at DSP214. The plotted voltage level, can be either the X component, that isthe in-phase component of the received signal, output of LIA 219, or theY component, that is the quadrature component of the received signal,output of LIA 219. The plot may be produced at the time the pipe istested or may be produced at a subsequent time from the data stored instorage 181.

[0102] The plot of FIG. 9 is a plot produced from a multiple frequenciesand provides a multi-frequency analysis of the pipe. In the plot of FIG.9, the multi-frequency analysis is performed using two selected separatefrequencies. Thus, there are two traces shown in the graph, eachrelating to a different frequency of a periodic signal output to thedriver coil. Trace 222 is a trace produced at a first selected frequencyand trace 224 is a second trace produced at a second selected frequency,which differs from the first frequency. The frequencies are chosen suchthat the slope of the traces produced by a broken wire shows a reversalwhen the same pipe section is surveyed by the two different frequenciesbut the traces produced by other features of the pipe do not result in aslope reversal. In the example of the graph of FIG. 9, the responserepresented by line 222 is a response corresponding to a sine wavedriver signal at frequency of 85 Hz. The trace produced at 224 is theresponse corresponding to a sine wave driver signal at a frequency of 35Hz.

[0103] The response of the detector is different at the two frequenciesand the differences in the response produced by the detector apparatusat the two frequencies provides information to determine whetheranomalies such as wire breaks are present in the helically extendingtensioned wires 102 of the pipe. The frequencies depicted in the graphplots are selected from the range of frequencies at which the magneticinspection test of the pipe was conducted. Naturally, the range offrequencies that a pipe was tested at may be many more than those thatare subsequently used to produce a particular graph. The response towire breaks shown in the region labelled 226 has a positive slopingexcursion for the 222 trace and a negative slope extending excursion forthe 224 trace. The reverse in sign of the slope at the differingfrequencies provides an indication that a wire break is present in theregion of 226. The response from wire breaks to frequencies, chosen inthis way, results in a trace pair that has diverging excursion slopeswhere wire breaks exist, but non-diverging slopes where other featuresexist, such as pipe joints. The traces may form a mirror image excursionin the region of the anomaly as depicted in region 226 of the trace. Theresponse in the region at 228 shows similar mirror image excursion andreversed sign slopes that indicate a wire break anomaly. The driver coiland detector are in a plane substantially orthogonal to the axis of thepipe under inspection. Region 230 of the graph of FIG. 8 shows aresponse when the detector passes through the region of a bell andspigot pipe joint, which provides a consistent excursion of the tracesfor each frequency. That is, the excursion slope of each trace has thesame sign. Each has a positive sloping excursion, or each has a negativesloping excursion.

[0104] The response trace of the detector apparatus to a wire breakprovides a diverging signal response trace at selected differentfrequencies in the region when a wire break is located. However, whenthe detector passes over a pipe joint, the selected differentfrequencies produce excursions with each excursion having the same signslope. In this manner, the driving signal frequencies can be used toproduce traces that distinguish between wire breaks and pipe joints.

[0105] The manner of supplying differing frequency driving signals toperform a magnetic inspection test of a pipe can be achieved by severalmethods of operation of the inspection apparatus. In one method ofoperation, the detector apparatus is passed through the pipe to beinspected several times. Each pass has a different frequency that istested. In a first pass a first driving frequency produces one trace,for example one of the traces appearing in the graph of FIG. 8. Thedetector apparatus is repositioned to the same start position and asecond pass occurs at a frequency different than the first. In theexample shown in FIG. 7 of the drawings, a first pass was made at 35 Hz,the vehicle was repositioned to the start position and a second pass wasperformed at the 85 Hz driver frequency.

[0106] Another manner of operating the apparatus is to perform a pass ineach direction at a different frequency. In this manner of operation, afirst pass along the pipe length occurs at a first frequency. When theend of the course of traverse of the pipeline that the inspection is tobe performed for has been reached, the driving frequency is changed tothe second desired frequency. From the end position, the detectorapparatus moves in a reverse direction back toward the start point anddetection is performed at a second driver frequency. In this manner,each traversal of the detector apparatus through the pipeline in eithera forward direction or a reverse direction will produce a trace. Thus,in this manner of operation one half of the number of traversals isrequired than would be required by operating the apparatus in a forwarddirection only.

[0107] A third manner of operation of the detector apparatus is toadvance the detector apparatus to discrete locations within the pipe. Ateach discrete location, the driver signal is swept over a range offrequencies or stepped through the various frequencies that are to beused in the pipe inspection. In this manner of operation, the detectorapparatus is advanced incrementally and at each test location, the testfrequency is to the desired settings. To produce the trace of FIG. 8 inthis manner of operation, the detector apparatus is positioned at afirst position. At that position, the variable frequency signalgenerator 204 is operated to produce a periodic signal at 35 Hz and thedetector signal is captured. The periodic driving signal is set to thenext frequency, 85 Hz, and the detector signal is then captured. Whenall of the periodic driving signal frequencies of interest have beenproduced at the position, the detector apparatus is then advanced to thenext position. At the next location, the periodic driver signalfrequency cycle is repeated. Operating the detector apparatus in thismanner requires only a single traversal of the pipe section to beinspected. At the conclusion of the traversal, data points are collectedto produce all of the traces representative of all the frequencies thatthe detection occurred at during the course of the traversal of thepipe.

[0108] A fourth method is to move the inspection apparatus through thepipe continuously while continuously changing the frequency of thedriver apparatus. When the frequency of the driver apparatus is changedsufficiently rapidly relative to the velocity of the inspectionapparatus, all of the frequencies of interest are applied every fewinches of displacement along the pipe. Traces representing eachindividual component frequency of interest can be produced from datarecorded during the inspection process. Thus, for example, the frequencycan be varied continuously from 100 Hz to 200 Hz over a period of 1second and data can be recovered at a sampling rate of 15,000 points persecond. The when it is desired to view the results at a particularfrequency, the sampling points that were captured at that frequency orthe closest available frequency can be viewed.

[0109]FIG. 10 is a graph showing traces resulting from traversal of apipe section selected from a plurality of different periodic drivingfrequencies extending over the range of 20 to 300 Hz, namely 24different periodic driving frequencies. Where a graph showing aplurality of periodic driving frequencies is produced, it is preferableto select a frequency separation of each periodic driving frequency fromanother such that the range of frequencies extends at least over oneoctave and the individual frequencies are separated from each other byat least one eighth of an octave. For example, use of a range of atleast 2 octaves will enable selection of about 17 frequencies, whereeach separated by one eighth of an octave. Wider separation ofindividual frequencies than one eighth of an octave will produce usefulresults but will reduce the number of frequency traces from 17 over therange. Conversely, narrower separation of individual frequencies thanone eighth of an octave for each trace will increase the number offrequencies plotted from 17 over the range. However, the individualtraces produced by the narrower separated individual frequencies may notprovide significant additional differences to warrant use of suchnarrower selected frequency separation.

[0110] In the traces of the graph of FIG. 9, a wire break is manifestedat 240, which show a plurality of signal trace or plots that havepositive sloping and negative sloping excursions in the range of 240.Thus, there is a sign reversal in the slopes of the excursions of thevarious traces in the region of 204, which is indicative of the presenceof a wire break at that location. When performing the test to producedata for production of the graphs, it is desirable to obtain test datapoints for a particular frequency at a particular location as quickly aspossible. The less time taken to gather data for each data point, willincrease the number data points available from a pipe inspection testsession over a given time period.

[0111]FIG. 11 is graph showing a plot of a component of a detectoroutput produced by the apparatus of the invention for a single periodicdriving frequency. The plot of FIG. 11 represents a trace produced at asingle driving frequency over the traversal of the pipe underinspection. The plot shows the output of either an X or Y component ofthe output of LIA 219 as the vertical axis for locations along the pipeas the horizontal axis of the plot. Large excursions 300 and 302 occurwhere the detector apparatus of the invention crosses over a pipe joint.Relatively smaller excursions 304 are representative of an anomaly whichmay warrant further consideration, as will be described in more detailwith reference to FIG. 12.

[0112]FIG. 12 is graph showing a plot of a component of a detectoroutput and a corresponding plot of a phase shifted component of adetector output produced by the apparatus of the invention for a singleperiodic driving frequencies. One plot of FIG. 11 is a plot of acomponent of a detector output 306 corresponding to the plot depicted inFIG. 11. The plot may be produced from either the X, or in-phase,component or the Y, or quadrature, component of the output of LIA 219provided on lines 221 or 223 of FIG. 8. The corresponding outputs X or Yfor the vector of the received signal are as described and shown withreference to FIG. 8a. The vector of the received signal may betransposed by an angle alpha, the process of which is described in moredetail with reference to FIG. 13. Transposition of the component by anangle alpha results in a second plot 308 of the transposed correspondingcomponent (which is either the in-phase or quadrature component). Theangle of transposition, alpha, is selected to provide a slope reversalof an anomaly of interest as shown in the region of the plot at the areareferenced by numeral 310. Production of the plot 308 based ontransposition of the detector vector by the transposition angle alpharesults in a mirror image form of plot only at the region of interest,namely region 310 which corresponds to an anomaly. The larger excursionsoccurring at known bell and spigot pipe joints at areas 312 of the plotdo not provide a mirror image plot in the transposition plot asillustrated when referring the two plots 306 and 308 in the regions 312.Thus selection of the transposition angle alpha is made such that theknown anomalies occurring at a bell and spigot pipe joint do not resultin mirror image excursions between the recorded plot 306 and thetransposed plot 308, calculated based on the transposition angle alpha.

[0113]FIG. 13 is a graph of a vector output representing the in-phaseand quadrature components of a received signal output from a lock-inamplifier. The in-phase axis I has a component value X corresponding tothe received vector and the quadrature axis Q has a component value Ycorresponding to the received vector. The vector may be described inpolar co-ordinates as having a length R and a phase theta relative tothe driving frequency. The vector may be transposed by an angle alpha,which will cause the in-phase and quadrature components of the receivedvector to assume new values. FIG. 13 illustrates the transpositiontransformation of one vector or data point pair by an angle alpha. Thistransposition is performed against all logged data point pairs of a dataset to produce the corresponding trace 308, which is transposed by anangle alpha as depicted in FIG. 12.

[0114]FIG. 14a is a cross section through a pre-stressed concrete pipe,generally indicated by 10, showing schematically a preferred arrangementof the driver and detector positioned exterior to, or around, the pipeunder test. The pre-stressed concrete pipe typically has an inner metalcylinder 11. Depending upon the type and grade of pipe, eitherpre-stressing wires are wound directly onto the cylinder, or a layer ofconcrete 13 is cast onto the cylinder, and the pre-stressing wires 12are wound on the layer of concrete. As noted previously, some pipes alsohave a layer of concrete cast inside the pipe, separating the metalcylinder from the interior volume of the pipe. Generally, another layerof concrete or protective mortar is cast around the wires to completethe pipe.

[0115] The pipe inspection apparatus is shown disposed on the exteriorof the pipe and comprises a driver coil 17 and a detector 16. Thedetector is preferably a coil detector capable of detecting magneticallyinduced currents in the pipe under inspection. The detector is adaptedto receive magnetic flux and convert it into a measurable electricalcurrent and voltage. The detector 16 is placed proximal to the exteriorsurface of the pipe. It is preferred that the detector does not touchthe pipe surface, as this would impede movement of the detector alongthe exterior surface of the wall. However, the gap between the detector16 and wall of the pipe 10 should be kept as small as is convenientlypossible, having regard for the fact that the detector is to be movedalong the length of the pipe.

[0116] Reference numeral 19 represents a diameter of the pipe, whichpasses through detector 16 in the arrangement of FIGS. 14a and 14 b. Inthis arrangement, driver coil 17 is disposed at the opposite end of thediameter 19 from detector 16. The driver coil is driven with the samelow frequency alternating current that has been previously described. Aspreviously described, it is preferable that the driver be placed asclose as conveniently possible to the wall of the pipe. Having regard tothe fact that the apparatus will be moved along the pipe, it is notdesirable to have the driver 17 dragging against the exterior wall ofthe pipe in operation of the apparatus.

[0117]FIG. 14b is an elevation view of the pipe and arrangement of FIG.14a, where the protective mortar of the pipe is not shown so that it ispossible to view the pre-stressing wires 12. For clarity, only a fewrepresentative wires are shown. The driver coil 17 is visible, but thedetector coil 16 is obscured behind pipe 10.

[0118]FIG. 15a is a cross section through a pre-stressed concrete pipe,showing schematically a alternate arrangement of the driver and detectoraround the pipe under test from the arrangement of FIG. 14a. The pipeinspection apparatus is shown disposed on the exterior of the pipe andcomprises a driver coil 17 and a detector 16. Each of the driver 17 andthe detector 16 is placed proximal to but not touching the exteriorsurface of the pipe as is driver coil 17. The driver and detector coilsare positioned so that the axis of the pipe (see 14 of FIG. 15b) isnormal to a line extending between the driver 17 and detector 16. Aspreviously described, the driver coil is driven with a low frequencyalternating current.

[0119]FIG. 15b is an elevation view of the pipe and arrangement of FIG.15a, in which the protective mortar of the pipe is not shown to enableviewing of the pre-stressing wires 12. For clarity, only a fewrepresentative wires are shown.

[0120]FIG. 16 is a top view of a pre-stressed water reservoir vessel250, showing schematically an arrangement of the driver and detectoraround the vessel under test. The inspection apparatus is shown disposedon the exterior of the reservoir vessel 250 and comprises a driver coil17 and a detector 16. Each of the driver 17 and the detector 16 isplaced proximal to but preferably not touching the exterior surface ofthe reservoir vessel 250 as is driver coil 17. The driver and detectorcoils are positioned so that the axis of the reservoir vessel (see 14 ofFIG. 15b) is normal to a line, shown in FIG. 16, extending between thedriver 17 and detector 16. As previously described, the driver coil isdriven with a low frequency alternating current. In operation of thearrangement of FIG. 16, the driver and detector are maintained in aspaced relationship and the apparatus is moved along the axis of thewater reservoir vessel 250. Multiple passes of the vessel can beperformed using a number of inspection processes. The apparatus cantraverse the water reservoir vessel under test. With each test, thedistance between the driver 17 and the detector 16 can be changed, orthe radial location of driver and detector apparatus around theperimeter of the water reservoir can be changed. Another variation is toprovide the driver at a fixed location and extend the receiver along aline of the perimeter of the water reservoir oriented parallel to theaxis of the water reservoir.

[0121]FIG. 17 is an elevation of the vessel and arrangement of FIG. 16with the pre-stressing wires 12 exposed for clarity.

[0122]FIGS. 18a, 18 b, 18 c and 18 d are cross sections through apre-stressed concrete cylinder showing schematically alternate preferredarrangements of the driver and detector disposed about the cylinderunder test. In these embodiments, the driver and detector are disposedon opposite sides of a cylinder under test. The arrangement of apparatusof FIGS. 18a through 18 d are less preferred as maintaining theorientation and relative position of the detector to driver as theapparatus extends along the cylinder under test is required. In thearrangement of each of FIGS. 18a through 18 d, the inspection apparatus,comprising a driver coil 17 and a detector 16, is shown disposed onproximal to but not touching a surface of the cylinder 10 to allowmovement of the apparatus along the surface of the cylinder.

[0123] In FIG. 18a, reference numeral 19 represents a diameter of thecylinder, which is parallel to the axis of driver 17 and passes throughdetector 16. In this arrangement, driver coil 17 is disposed on theexterior of cylinder 10 along the diameter 19 from detector 16, which isdisposed on an interior side of cylinder 10.

[0124] In FIG. 18b, driver 17 is disposed on the exterior side ofcylinder 10 under test. The axis of driver 17 is oriented towarddetector 16 such that the line extending between the driver 17 anddetector 16 is normal or orthogonal to the axis of the cylinder underinspection. In this arrangement, driver coil 17 is disposed on theexterior of cylinder 10 and detector 16 is disposed on an interior sideof cylinder 10.

[0125] In FIG. 18c, reference numeral 19 represents a diameter of thecylinder, which is parallel to the axis of driver 17 and passes throughdetector 16. In this arrangement, driver coil 17 is disposed on theinterior of cylinder 10 along the diameter 19 from detector 16, which isdisposed on the exterior side of cylinder 10.

[0126] In FIG. 18d, driver 17 is disposed on an interior side ofcylinder 10 under test. The axis of driver 17 is oriented towarddetector 16 such that the line extending between the driver 17 anddetector 16 is normal or orthogonal to the axis of the cylinder underinspection. In this arrangement, driver coil 17 is disposed on aninterior side of cylinder 10 and detector 16 is disposed on an exteriorside of cylinder 10. The line extending between driver 17 and detector16 is normal or orthogonal to the axis of the cylinder 10 under test.

[0127] While the invention has been shown with respect to certainembodiments, it will be understood that many variations to suchembodiments will be evident to a person skilled in the art, and it isintended that all such evident variations should be protected.

We claim:
 1. An inspection apparatus for detecting discontinuities intensioning wires of a concrete pipe, comprising: (i) driver means toinduce a current in a metallic pre-stressing reinforcement of a concretepipe; (ii) detector means to produce an output responsive to magneticflux, the detector means spacedly disposed from said driver coil andadapted to be disposed proximal to a surface of the pipe and notdisplaced axially along the pipe from said driver means more than onepipe diameter from a plane orthogonal to the axis of the pipe and commonto said driver means; (iii) displacement sensor means to output thedistance of the inspection apparatus from a known location; (iv) meansfor causing the inspection apparatus to move along the concrete pipe;and (v) means for storing outputs corresponding to detector means andthe displacement sensor means.
 2. The inspection apparatus as claimed inclaim 1, in which the detector means is a coil adapted to be disposed inrelation to the concrete pipe with an axis parallel to the axis of theconcrete pipe, and the output is a voltage induced in the coil by saidmagnetic flux.
 3. The inspection apparatus as claimed in claim 1, inwhich the detector means is a pair of coils adapted to be disposed inrelation to the concrete pipe such that each has an axis parallel to theaxis of the concrete pipe, and the output is a voltage induced in thecoils by said magnetic flux.
 4. The inspection apparatus as claimed inclaim 1, in which the detector means comprises at least one giantmagnetoresistive sensor and the output is a change in resistance inducedin the magnetoresistive sensor by said magnetic flux.
 5. The inspectionapparatus as claimed in claim 1, in which the detector means is a pairof giant magnetoresistive sensors and the output is a change inresistance induced in the said magnetoresistive sensors by said magneticflux.
 6. The inspection apparatus as claimed in claim 1, in which thedetector means is a plurality of spacedly disposed giantmagnetoresistive sensors, each oriented to be responsive to a magneticflux of a corresponding region of each one such giant magnetoresistivesensor, and the output is a change in resistance induced in the saidmagnetoresistive sensors by said magnetic flux.
 7. The inspectionapparatus as claimed in claim 1 wherein said driver means comprises acoil having an axis that is oriented radial to the pipe.
 8. Theinspection apparatus as claimed in claim 7, in which the driver coil islocated diametrically across the pipe from said detector means.
 9. Theinspection apparatus as claimed in claim 1, in which the driver coil isoffset circumferentially from the detector means by an angle of at least10 degrees.
 10. The inspection apparatus as claimed in claim 1, in whichthe driver coil is offset around the circumference of the pipe from thedetector means by a distance of at least one meter.
 11. The inspectionapparatus as claimed in claim 1 further including a magnetic shieldinterposed between the driver coil and the detector means.
 12. Theinspection apparatus of claim 1 mounted on a vehicle suited to movementthrough a pipeline.
 13. The inspection apparatus of claim 1 additionallyincluding means for converting the force of flow of a liquid in thepipeline into motion of the vehicle.
 14. The inspection apparatus asclaimed in claim 13, further including means for adjusting such meansfor converting the force of flow operative to slow or stop the vehiclewhen desired.
 15. A method of detecting discontinuities in tensioningwires of a concrete pipe comprising: (i) providing a driving signal to adriver having an axis oriented orthogonal to an axis of a concrete pipeand proximal to an inside surface thereof to generate an induced currentin pre-stressing elements extending substantially circumferentially ofthe pipe; (ii) providing a detector for producing an output responsiveto a magnetic flux, the detector located in close proximity to aninterior wall of a pipe and axially displaced not more than one pipediameter of a plane orthogonal to the axis of the pipe and common to thedriver, (iii) moving the detector along the wall of the pipe; and (iv)recording the output and the location of the detector as it moves.
 16. Amethod of testing the tensioning wires of a pre-stressed concrete pipealong a length thereof using apparatus including magnetic fluxproduction means spacedly disposed from magnetic flux detector means,said magnetic flux detector means adapted to be disposed proximal to asurface of a pipe, the magnetic flux production means producing amagnetic field in response to a driving signal and the detector meansproducing a detector signal in response to magnetic flux and locatingmeans to indicate a location along said pipe and control meansoperatively connected to said locating means, to said magnetic fluxproduction means and to said magnetic flux detector means, the methodcomprising performing the steps of: (i) providing a driving signal of atleast one frequency; (ii) receiving a detector signal; and (iii)recording the detector signal in relation to the frequency of thedriving signal and the location indication over a range of locationstraversed along a length of pipe.
 17. The method of claim 16 furtherincluding the steps of: (i) selecting at least one frequency, and (ii)producing a trace for each selected frequency of the recorded detectorsignal in relation to the frequency of the driving signal and theselected location upon a display means.
 18. The method of claim 17wherein the display means is a display device.
 19. The method of claim17 wherein the display means is a printed graph.
 20. The method of claim16 wherein one frequency of driving signal is provided over the range oflocations traversed.
 21. The method of claim 20 further including thestep of: (i) providing a start location and an end location definingsaid range of locations along a length of pipe; (ii) providing a singlefrequency of driving signal; and (iii) traversing said range oflocations advancing from the start location to the end location.
 22. Themethod of claim 20 further including the steps of: (i) providing a startlocation and an end location defining said range of locations along alength of pipe; (ii) providing a range of frequencies of said drivingsignal; (iii) traversing the range of frequencies at each location ofsaid range of locations from the end location to the start location. 23.The method of claim 16 wherein the detector signal in relation to eachfrequency of the driving signal and the location indication is recordedat each location before traversing to a successive selected location.24. A method of testing tensioning wires of a pre-stressed concrete pipealong a length thereof using apparatus including magnetic fluxproduction means and magnetic flux detector means each disposed proximalto a surface of a pipe to be tested in a spaced relationship to theother and axially disposed within one pipe diameter to the other, themagnetic flux production means producing a magnetic field in response toa driving signal and the detector means producing a detector signal inresponse to magnetic flux, location indication means and control meansoperatively connected to said location indication means, to saidmagnetic flux means and to said detector means, the method comprising:(i) providing a driving signal of at least one frequency; (ii) receivinga detector signal; (iii) producing an output representative of thedetector signal corresponding to the in-phase and quadrature componentsof the detector signal in relation to the fundamental frequency of thedriving signal; and (iv) recording the output representative of thedetector signal in relation to the fundamental frequency and thelocation; whereby at least one fundamental frequency is recorded over arange of locations traversed along a length of pipe.
 25. The method ofclaim 24 including the step of producing an output representative of theamplitude and the phase of the detector signal expressed in Cartesianco-ordinates.
 26. The method of claim 24 further includes the steps ofdisplaying a first trace for at least one selected frequency of therecorded detector signal and the selected location upon a display meansand displaying a second trace calculated from the Cartesian co-ordinatesof the first trace based on a transposition angle alpha.
 27. The methodof claim 26 wherein the transposition angle alpha is selected to producea mirror image excursion of portions of each trace over a range ofinterest.
 28. The method of claim 27 wherein the display means is adisplay device.
 29. The method of claim 27 wherein the display means isa printed graph.
 30. The method of claim 24 wherein for each frequencyof the driving signal, a range of locations are displayed.
 31. Themethod of claim 24 wherein the length along the pre-stressed concretepipe is a range of locations defined by a start location and an endlocation and the pipe is traversed from the start location to the endlocation for each selected frequency.
 32. An inspection apparatus fordetecting discontinuities in tensioning wire of a concrete cylinder,comprising: (i) a driver coil adapted to be oriented with an axisorthogonal to an axis of a concrete cylinder to induce a current in atensioning wire of a concrete cylinder in response to a driving signal;(ii) means for producing a driving signal; (iii) detector apparatus toproduce an output responsive to a magnetic flux, said detector apparatushas an axis orthogonal to the axis of said driver coil and spacedlydisposed there from, said detector coil adapted to be oriented proximalto a surface of said concrete cylinder and substantially in a planeorthogonal to an axis of the concrete pipe in common with the drivercoil; and (iv) filter means for producing an output signalrepresentative of differences between the driving signal and the outputproduced by said detector apparatus.
 33. The apparatus as claimed inclaim 32, in which the detector apparatus is a coil adapted to bedisposed with an axis parallel to the axis of the cylinder, and theoutput is a voltage induced in the coil by said magnetic flux.
 34. Theapparatus as claimed in claim 32, in which the detector apparatus is apair of coils each adapted to be disposed with an axis parallel to theaxis of the cylinder, and the output is a voltage induced in the coilsby said magnetic flux.
 35. The apparatus as claimed in claim 32, inwhich the detector apparatus is a giant magnetoresistive sensor and theoutput is a change in resistance induced in the magnetoresistive sensorby said magnetic flux.
 36. The apparatus as claimed in claim 32, inwhich the detector apparatus is a pair of giant magnetoresistive sensorsand the output is a change in resistance induced in the saidmagnetoresistive sensors by said magnetic flux.
 37. The apparatus asclaimed in claim 32, in which the detector apparatus is a plurality ofspacedly disposed giant magnetoresistive sensors, each oriented to beresponsive to a magnetic flux of a corresponding orthogonal axis to eachother one such giant magnetoresistive sensor, and the output is a changein resistance induced in each said magnetoresistive sensors by saidmagnetic flux.
 38. The apparatus as claimed in claim 32 wherein saiddriver means comprises a coil adapted to be oriented with an axis thatis oriented radial to the cylinder.
 39. The apparatus as claimed inclaim 38, in which the driver coil is spacedly disposed from saiddetector apparatus to be adapted to be positioned diametrically acrossthe concrete cylinder from said detector apparatus.
 40. The apparatus asclaimed in claim 32, in which the driver coil is spacedly disposed fromsaid detector apparatus to be adapted to be positioned circumferentiallyoffset from the detector apparatus by an angle of at least 10 degrees.41. The apparatus as claimed in claim 32, in which the driver coil isspacedly disposed from said detector apparatus to be positionedcircumferentially offset from the detector apparatus by a distance of atleast one meter.
 42. The apparatus as claimed in claim 32 furtherincluding a magnetic shield interposed between the driver coil and thedetector apparatus.
 43. The apparatus as claimed in claim 32 furtherincluding: (i) displacement sensor means to produce an outputrepresentative of at least one distance of the detector apparatus a froma known location; (ii) means to cause the detector apparatus to move;and (iii) means for storing outputs derived from the output of thefilter means and said displacement sensor means.
 44. The apparatus ofclaim 32 wherein said filter apparatus includes a multiplier and a lowpass filter.
 45. The apparatus to claim 32 wherein said filter apparatusincludes a lock-in amplifier.
 46. The apparatus of claim 32 mounted on avehicle suited to movement along a pipeline.
 47. A method of detectingdiscontinuities tensioning wires a concrete cylinder comprising: (i)providing a driving signal to a driver coil means having an axisoriented orthogonal to an axis of a concrete cylinder to generateinduced current in pre-stressing elements extending substantiallycircumferentially around the cylinder; (ii) providing a detectorproximal to a surface of the concrete cylinder and spacedly disposedfrom said driver means along said axis of said driver coil means, thedetector to produce an output responsive to a magnetic flux; (iii)filtering the output of the detector relative to the driving signal; and(iv) recording the filtered output and the location of the detector. 48.A method of testing the tensioning wires of a pre-stressed concretecylinder along a length thereof using apparatus including magnetic fluxproduction means and magnetic flux detector means each proximal to asurface of a cylinder and in a spaced relationship to each other, themagnetic flux production means to produce a magnetic field in responseto a driving signal and the detector means to produce a detector signalin response to magnetic flux and locating means to indicate a locationalong said cylinder and control means operatively connected to saidlocating means, to said magnetic flux means and to said detector means,the method comprising performing the steps of: (i) providing a drivingsignal of at least one frequency; (ii) receiving a detector signal;(iii) filtering the received detector signal relative to the drivingsignal; (iv) receiving a location indication; and (v) recording thefiltered detector signal, the frequency of the driving signal and thelocation indication over a range of locations traversed along a lengthof cylinder.
 49. The method of claim 48 further including the steps ofselecting at least one frequency and displaying a trace for eachselected frequency of the recorded detector signal in relation to thefrequency of the driving signal and the selected location upon a displaymeans.
 50. The method of claim 49 wherein the display means is a displaydevice.
 51. The method of claim 49 wherein the display means is aprinted graph.
 52. The method of claim 48 wherein one frequency ofdriving signal is provided over the range of locations traversed. 53.The method of claim 52 further including the step of: (i) providing astart location and an end location defining said range of locationsalong a length of cylinder; (ii) providing a single frequency of drivingsignal; and (iii) traversing said range of locations advancing from thestart location to the end location.
 54. The method of claim 52 furtherincluding the steps of: (i) providing a start location and an endlocation defining said range of locations along a length of cylinder;(ii) providing a range of frequencies of said a driving signal; (iii)traversing the range of frequencies at each location of said range oflocations from the end location to the start location.
 55. The method ofclaim 48 wherein the filtered detector signal in relation to eachfrequency of the driving signal and the location indication is recordedat each location before traversing to a successive selected location.56. A method of testing the tensioning wires of a pre-stressed concretecylinder along a length thereof using apparatus including magnetic fluxproduction means and magnetic flux detector means each disposed proximalto a surface of the cylinder in a spaced relationship to the other andaxially disposed not more than one cylinder diameter from the other, themagnetic flux production means for producing a magnetic field inresponse to a driving signal and the detector means for producing adetector signal in response to magnetic flux, location indication meansand control means operatively connected to said location indicationmeans, to said magnetic flux means and to said detector means, themethod comprising: (i) providing a driving signal of at least onefrequency; (ii) receiving a detector signal; (iii) producing an outputrepresentative of the detector signal corresponding to the in-phase andquadrature components of the detector signal in relation to thefundamental frequency of the driving signal; and (iv) recording theoutput representative of the detector signal in relation to thefundamental frequency, the fundamental frequency and the location;whereby at least one fundamental frequency is recorded over a range oflocations traversed along a length of cylinder.
 57. The method of claim56 wherein the step of producing an output representative of theamplitude and the phase of the detector signal comprises formingCartesian co-ordinates X and Y.
 58. The method of claim 56 furtherincluding the steps of displaying a first trace for at least oneselected frequency of the recorded detector signal and the selectedlocation upon a display means and displaying a second trace calculatedfrom either the X or Y co-ordinates of the first trace based on atransposition angle alpha.
 59. The method of claim 58 wherein thetransposition angle alpha is selected to produce a mirror imageexcursion of portions of each trace over a range of interest.
 60. Themethod of claim 59 wherein the display means is a display device. 61.The method of claim 59 wherein the display means is a printed graph. 62.The method of claim 56 wherein for each frequency of the driving signal,a range of locations are displayed.
 63. The method of claim 56 whereinthe length along the pre-stressed concrete cylinder is a range oflocations defined by a start location and an end location and thecylinder is traversed from the start location to the end location foreach selected frequency
 64. A method of testing the tension wires of apre-stressed concrete cylinder along a length thereof using apparatusincluding magnetic flux production means and magnetic flux detectormeans each disposed proximal to a surface of the cylinder, the magneticflux production means for producing a magnetic field in response to adriving signal and the detector means for producing a detector signal inresponse to magnetic flux, location indication means and control meansoperatively connected to said location indication means, to saidmagnetic flux means and to said detector means, the method comprising:(i) providing a driving signal of at least one frequency; (ii)positioning the detector means in relation to the magnetic fluxproduction means, each said detector means and the magnetic fluxproduction means axially disposed not more than one cylinder diameter tothe other; (iii) receiving a detector signal; (iv) producing an outputrepresentative of the detector signal corresponding to the in-phase andquadrature components of the detector signal in relation to thefundamental frequency of the driving signal; and (v) recording thelocation of the detector means and output representative of the detectorsignal in relation to the fundamental frequency, the fundamentalfrequency and the location of a deter; whereby at least one fundamentalfrequency is recorded over a range of locations traversed by saiddetector means along a length of a pre-stressed concrete cylinder. 65.The method of claim 64 wherein producing an output representative of theamplitude and the phase of the detector signal comprises Cartesianco-ordinates X and Y.
 66. The method of claim 64 further includes thesteps of producing a first trace for at least one selected frequency ofthe recorded detector signal and the selected location upon a displaymeans and producing a second trace calculated from either the X or Yco-ordinates of the first trace based on a transposition angle alpha.67. The method of claim 66 wherein the transposition angle alpha isselected to produce a mirror image excursion of portions of each traceover a range of interest.
 68. The method of claim 67 wherein the displaymeans is a display device.
 69. The method of claim 67 wherein thedisplay means is a printed graph.
 70. The method of claim 64 wherein foreach frequency of the driving signal, a range of locations of saiddetector means with respect to the axis of the concrete cylinder aredisplayed.
 71. The method of claim 64 wherein the length along thepre-stressed concrete cylinder is a range of locations defined by astart location and an end location and the cylinder is traversed by saiddetector means from the start location to the end location for eachselected frequency.
 72. Apparatus for detecting discontinuities in aspirally wound metallic pre-stressing reinforcements embedded the wallof a concrete pipe or substantially cylindrical fluid containmentstructure having a longitudinal axis, comprising: (i) a detectororiented to lie along the wall surface of said concrete pipe orstructure to be monitored, said detector being capable of detectingmagnetic flux flowing in a direction parallel to the longitudinal axisof the pipe or structure, means for causing the detector to move alongthe surface of such pipe or structure; (ii) means for determining atleast one of the location of the detector or the distance it has movedfrom a known original location; (iii) means for recording the magneticflux recorded by the detector and recording it in association with thelocation of the detector when such flux was detected or the distancethat the detector has moved from an original known location when suchflux was detected.
 73. Apparatus as claimed in claim 72, in which thedetector is a coil having an axis parallel to the longitudinal axis ofthe pipe, and the magnetic flux is detected as a voltage induced in thecoil.
 74. Apparatus as claimed in claim 72, in which the detector is aGMR sensoe.
 75. Apparatus as claimed in any of claims 72-74, includingmeans for generating a periodically varying current in the spirallywound metallic pre-stressing reinforcements.
 76. Apparatus as claimed inany of claims 72-74, additionally comprising a driver coil having anaxis which is radial to the longitudinal axis, for generating suchperiodically varying current.
 77. Apparatus as claimed in claim 76, inwhich the driver coil is located diametrically across the pipe orstructure from the detector.
 78. Apparatus as claimed in claim 76, inwhich the driver coil is located diametrically across the pipe orstructure from the detector, but axially offset therefrom a distance notmore than one diameter of the pipe or structure.
 79. Apparatus asclaimed in claim. 75, in which the driver coil is offsetcircumferentially from the detector on the pipe or structure by an angleof at least 90 degrees.
 80. Apparatus as claimed in any of claims 75-79,having a shield for magnetic flux located in line of sight between thedetector and the driver coil.
 81. Apparatus comprising the apparatus ofany claims 72-80, where the pipe or structure is a pipeline and theapparatus is mounted on a vehicle suited to movement through a pipeline.82. Apparatus of claim 81 additionally including means for transmittingthe force of flow of liquid in the pipeline into forward motion of thevehicle.
 83. Apparatus as claimed in claim 82, additionally comprisingmeans for adjusting such means for transmitting the force of flow,whereby to slow or stop the vehicle when desired.
 84. A method ofdetecting discontinuities in spirally wound metallic pre-stressingelements of a concrete pipe having such elements which comprises: (i)providing a detector coil which has its axis parallel to the axis of thepipe, and which is of a diameter less than one quarter of the diameterof the pipe, (ii) placing the detector coil in close proximity to theinterior wall of the pipe, (iii) generating a periodically varyinginduced current in the pre-stressing elements, (iv) moving the detectorcoil along the wall of the pipe and (v) recording the distance moved orthe location of the detector coil, and the induced voltage or currentsensed by the detector coil as it moves.
 85. A method as claimed inclaim 84, in which the periodically varying induced current is generatedby a driver coil orthogonal to the detector coil.
 86. A method asclaimed in claim 84 or claim 85, in which the periodic current is asinusoidal waveform.
 87. A method as claimed in claim 84 or claim 85, inwhich the periodic current has a sawtooth waveform.
 88. A method asclaimed in claim 84 or claim 85, in which the periodic current has asquare waveform.